G protein-coupled receptors (GPCRs) are cell surface proteins that translate hormone or ligand binding into intracellular signals. GPCRs are found in all animals, insects, and plants. GPCR signaling plays a pivotal role in regulating various physiological functions including phototransduction, olfaction, neurotransmission, vascular tone, cardiac output, digestion, pain, and fluid and electrolyte balance. Although they are involved in various physiological functions, GPCRs share a number of common structural features. They contain seven membrane domains bridged by alternating intracellular and extracellular loops and an intracellular carboxyl-terminal tail of variable length.
GPCRs have been implicated in a number of disease states, including, but not limited to cardiac indications such as angina pectoris, essential hypertension, myocardial infarction, supraventricular and ventricular arrhythmias, congestive heart failure, atherosclerosis, renal failure, diabetes, respiratory indications such as asthma, chronic bronchitis, bronchospasm, emphysema, airway obstruction, upper respiratory indications such as rhinitis, seasonal allergies, inflammatory disease, inflammation in response to injury, rheumatoid arthritis, chronic inflammatory bowel disease, glaucoma, hypergastrinemia, gastrointestinal indications such as acid/peptic disorder, erosive esophagitis, gastrointestinal hypersecretion, mastocytosis, gastrointestinal reflux, peptic ulcer, Zollinger-Ellison syndrome, pain, obesity, bulimia nervosa, depression, obsessive-compulsive disorder, organ malformations (for example, cardiac malformations), neurodegenerative diseases such as Parkinson's Disease and Alzheimer's Disease, multiple sclerosis, Epstein-Barr infection and cancer.
The magnitude of the physiological responses controlled by GPCRs is linked to the balance between GPCR signaling and signal termination. The signaling of GPCRs is controlled by a family of intracellular proteins called arrestins. Arrestins bind activated GPCRs, including those that have been agonist-activated and especially those that have been phosphorylated by G protein-coupled receptor kinases (GRKs).
The abnormal regulation of hormones that bind to G protein-coupled receptors underlies the pathogenesis of many diseases. The ability to measure serum and tissue levels of these regulators, while clinically and scientifically desirable, is presently limited to very specialized biochemical and immunochemical assays.
Altered concentrations of a GPCR ligand in a biological sample may be indicative of a disease state. Altered concentrations of a GPCR ligand in an environmental sample may indicate the presence of harmful chemicals. There is a need for highly sensitive and specific methods for the quantitative detection of GPCR ligands in a heterogeneous sample, as well as methods for the detection of the multiple bioactive isoforms of a GPCR ligand in a heterogeneous sample. Sensitive, rapid methods of analyzing the presence of GPCR ligands in heterogeneous samples, both biological and environmental, would improve disease diagnosis and the detection of harmful compounds in the environment.